Tandem Fork?

[WVUCyclist]

So I'm converting my co-motion tandem to 148 boost and building a new fork for it to go with a 110 boost front hub. I couldn't find much on tandem fork blades or steerer tubes, so I'm thinking of going with a segmented fork with the following specs:

Steerer tube 1 1/8" 4130 0.120 wall

Fork blades and segments 1" 4130 0.083 wall

Paragon hooded dropout

Any data to suggest this is appropriate or not?

Thanks,

Gef Moy

Colorado Springs, Co

[Jon Norstog]

Well, those blades should be good and stiff!

jn

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[Ethan Labowitz]

Does Waltworks still sell fork blades made for the specific job of segmented fork blades?

I've used them, they're great, might save some weight, I have no idea.

Ethan (he/they)
sent from mobile

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[WVUCyclist]

Yes, but not specific to tandem. The fork will be road/gravel disc, but I don't want to just assume that mtb fork blades will be sufficient for tandem. Maybe they are, but high consequence if it fails. I'd rather be stiff and overbuilt than needing dental work.

[Alex Wetmore]

Have you measured the steerer on your existing Co-Motion fork?  I bet it is pretty standard 1-1/8" steel with 2.3mm/1.6mm butting.

Have you asked Co-Motion what they would charge for a new fork?  It's fun to build things, but this is one case where they might have custom drawn tubes and you could end up with a significantly lighter option for a reasonable cost.

Alex

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From: frameb...@googlegroups.com <frameb...@googlegroups.com> on behalf of WVUCyclist <gef...@gmail.com>
Sent: Sunday, June 12, 2022 3:42 PM
To: Framebuilders <frameb...@googlegroups.com>
Subject: [Frame] Tandem Fork?

 

[Jon Norstog]

That is the best idea I have seen in this discussion.

jn

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[Ethan Labowitz]

I mean...which is stiffer, and which is stronger, in this application?:

- Waltworks blade of 28.6 mm OD and a tapering 1.2/0.8mm wall, vs.

- 25.4 mm OD, .083 str8 gauge

I'm not enough of the right kind of nerd to want to model that to get the answers, but it sounds relatively model-able for someone who is the right kind of nerd

My money is on the Waltworks blades. They must win for stiffness at least.

Partly I say this bc I built a fork out of some considerably weaker fork blades, which bent from doing some MTB nonsense: https://photos.google.com/share/AF1QipNm5iVRdfRMY-tmj3r52PShej9_bkqCQ_P4-Jk8El0R4sSVucV5gPEa7uDXPIlVRQ?key=M3l6QjlHVlZ6MWhqQklDLXNIaVk3a2gzMzhjX0t3

I had a trip planned for soon after the fork bent, so I needed some much burlier fork blades right away...I ordered some Waltworks, built this fork

https://photos.google.com/share/AF1QipOU3MAScR5NyBrtm2eYo7SrdlTqOEgOt1iUniknFhJqLbI6F3Vn3jytU94xTcsOTA?key=TVpqZ2thVGgyMUFYWEVSMmpPd2tsbTJ4SVlmNUtn

rode it for a season on a bikepacking rigid frame I also made

https://photos.google.com/share/AF1QipOJlmXQjyWlZSw8cs-wr0_3UNlKnl1PKaFhi2TibRAspXVoTag_2pwmCiSKmqlrdQ?key=R1hmS3dCQkJlNTlkbkY5T2ZoVW1vQkZ5U2lBdE1B 

It worked great til I wrecked the frame in some different MTB nonsense, which wasn't caused by the fork at all, I had a stupid chain issue that bent a chainstay badly.

I feel like if a person 

The Waltworks blades have been excellent, even on some nasty MTB bikepacking, heavily loaded, rough, rocky adventures. I think if I was building a tandem, I'd at very least consider them...tandems are generally road bikes to my knowledge, which has got to be so much easier on a frame/fork than MTB.

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Ethan (he/they)

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[WVUCyclist]

Ethan- Thanks for the pictures! What were the weaker blades? I want to agree that MTB is similar to road tandem, but that's a total guess on my part.

Quick calculations result in 
Yeild strength: 0.083 1" 1243 lbs

0.047 1.125" 1010 lbs

For a 1000 lb load

0.083 1" deflects 1.48"

0.047 1.125" deflects 1.62"

I wasn't able to model a butted tube, but this satisfies Waltworks vs thicker straight wall strength & stiffness. It doesn't answer tandem specific strengths required. It most certainly will be heavier.

 Alex, reached out to Comotion, I'll see what they say and I'll measure the existing fork (at least the steerer) when I get home. I do really get a kick out of building things and since it's a tandem I'm not terribly concerned about the weight.

[Colin Pinney]

Hey Gef, 

I don't know if it is the right fork for your purpose,  but I have used the True Temper Seat tube as tandem fork blades for 5 years, the predecessor to the blades you can get from Walt.  My daughter er and I have up over 1000 miles on it.  In March we rode Joe's loop at 18 road in Fruita on it.  Fork did great, I wish I had bigger disk brakes then 180, 160.

The wheel size is 26" x 2.1".

Together we weight around 270 on the bike.

The photo is probably the most I have weighed the bike down on a 3 day bike glamping trip.

Kindly

Colin

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[Hahn Rossman]

Gef

It's also important to know how long your blades are going to be. ATC is a big driver if why mountain bike forks fail at the top of the blade. The 1" fat chance forks ended up getting gussets on the back side and they weren't very long forks by modern standards.

Hahn Rossman

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[Ethan Labowitz]

They were 25.4mm OD, tapering to 17mm at the dropout ends. They were a special run made up by Nova for a builder in MA. The tops were a little thin on the wall--nominally 1.0mm, but actually a little under that. I don't have a measurement of the wall thickness at the dropout end.

Here's what I messaged a friend just after building the replacement fork with the Waltworks blades:

It could be placebo, or the design (blades at a different angle to horizontal), or the blades, but the new fork is dramatically better than the old one. It feels stiffer, which I usually consider a bad thing in a steel frameset, but in this case I mean it as a plus, in that the steering feels much more predictable, especially loaded up with bikepacking gear. I felt pretty comfortable on the new fork within a few hours of installing it, whereas it took far longer with the old fork to get to the same level of comfort.

Ethan (he/they)

[Alex Meade]

Addressing this question:

______________________

I mean...which is stiffer, and which is stronger, in this application?:

- Waltworks blade of 28.6 mm OD and a tapering 1.2/0.8mm wall, vs.

- 25.4 mm OD, .083 str8 gauge

______________________

Well, I guess I'm one of those nerds:

A 25.4mm straight tube with a 2.1mm wall compared to straight 28.6 mm with 0.8wall:  The 28.6 x 0.8 has 64% of the Area Moment of Inertia, so it's 64% as stiff as the 25.4 x 2.1.

A 25.4mm x 2.1 compared to straight 28.6 x 1.2mm: the 28.6 x 1.2 has 94% of the stiffness of the 25.4x2.1.

So, unless I'm misunderstanding something, the taper does't matter to the conclusion that the 25.4x 2.1 is the stiffer tube.  And someone correct me if I did my math wrong!

Alex

alexmeadetools.com

[Alistair Spence]

Alex,

I ran those numbers and got pretty much exactly the same results so I think you're right on the money here.

Second moment of area for 24.5mm x 2,1 wall tube = 10516 mm4 (not sure how to insert that 4th power as a superscript, sorry).

Second moment of area for 28.6mm x 1.2 wall tube = 9712 mm4

9712/10516 = 92.4%

Also, with the wall thickness of the 28.6mm tube tapering to 0.8mm, it will in practice be less than 92% as stiff as the 25.4mm tube.

Alistair,

Seattle

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Alistair Spence,

Seattle, WA.